Natural gas contains a wide range of species which are capable of forming solids during the cryogenic process of producing LNG known as liquefaction. One of the species that causes considerable problems to LNG producers is carbon dioxide. In a conventional LNG facility, pretreatment of the natural gas is conducted to decrease the carbon dioxide content to between 50 and 125 ppm prior to the natural gas entering the liquefaction process.
On average, carbon dioxide compositions in a natural gas feed stream can range between 0.5% and 30% mol and can be as high as 70% mol in commercially viable reservoirs like Natuna, Indonesia. Carbon dioxide is typically removed using chemical reactions such as reversible absorption processes with amine solvents. These are expensive and complex processes and commonly encounter operational problems such as foaming, corrosion, blocked filters and amine degradation. Losses of amine, water and hydrocarbons are commonly encountered. These processes also consume significant amounts of energy to regenerate and pump the solvent.
LNG refrigeration systems are expensive because so much refrigeration is needed to liquefy natural gas. A typical natural gas stream enters a LNG plant at pressures from about 40 bar to about 76 bar and temperatures from about 20° C. to about 40° C. Natural gas, which is predominantly methane, cannot be liquefied by simply increasing the pressure, as is the case with heavier hydrocarbons used for energy purposes. The critical temperature of methane is −82.5° C. This means that methane can only be liquefied below that temperature regardless of the pressure applied. Since natural gas is a mixture of gases, it liquefies over a range of temperatures. The critical temperature of natural gas is typically between about −85° C. and −62° C. Natural gas compositions at atmospheric pressure will typically liquefy in the temperature range between about −165° C. and −155° C. Since refrigeration equipment represents such a significant part of the LNG facility cost, cleaning of this equipment is important.
In conventional LNG plants, the natural gas is typically cooled in one or more heat exchangers. If insufficient carbon dioxide is removed prior to the natural gas entering the heat exchangers, carbon dioxide precipitates as a solid and accumulates on the cold surfaces of the heat exchangers and other plant equipment eventually rendering these items inoperable. When fouling has reached a critical level, the vessel must be taken off-line for the carbon dioxide solids to be removed. This can be achieved by warming-up the affected equipment. However, this causes considerable downtime and energy-loss for the plant. Alternatively, the solid carbon dioxide may be removed mechanically. In such a case of mechanical defouling of equipment, the vessel, baffles and/or pipework may be damaged, which only encourages further fouling in the next production cycle. Moreover, solids condensing on metal surfaces form an insulating film reducing the thermal efficiency of the heat exchanger.
There is a need for a simpler, more economical process for the removal of solid carbon dioxide that has fouled plant equipment under cryogenic conditions.